The present invention relates to a piezoelectric coupler for selectively coupling two bodies and a joint incorporating the coupler.
Piezoelectric material has a crystalline structure that axially expands or contracts when a voltage difference is applied along the axis. Whether the material expands or contracts depends on the polarity of the voltage difference. For example, a positive voltage difference may cause the material to expand and a negative voltage difference may cause the material to contract. The greater the voltage difference, the greater the deformation. When the voltage difference is removed the structure returns to its initial undeformed state.
Devices that utilize the ability of piezoelectric materials to selectively expand or contract are known. Such devices may form couplers or joints that selectively couple or join two bodies. The device is attached to one body. The piezoelectric material expands and extends to engage the other body and couple the two bodies together. The material contracts to uncouple the two bodies. The expansion or contraction of the piezoelectric material can be finely controlled by varying the applied voltage difference.
In one known type of piezoelectric device a number of piezoelectric elements are stacked together to form an element stack having an initial stack height. The elements are each connected to electrodes that apply a voltage difference from a voltage source to opposite sides of the element. The voltage difference increases the height of each element and so the overall stack height increases. The stack height is controlled by varying the voltage difference. The stack extends to engage the end of the stack against the other member and apply a coupling force to the other member.
Piezoelectric stacks have a number of disadvantages that often render them unsuitable for joining or coupling two bodies. The large number of electrodes requires complicated and expensive wiring. The element stack must be supported in a housing, with the electrodes and wiring contained in the housing. Such an assembly is heavy and takes up a large amount of space. The stack force is applied over a small area and often a number of stacks must operate simultaneously to apply sufficient coupling force.
Thus, there is a need for an improved piezoelectric coupler for selectively coupling two bodies. The improved coupler should be compact and lightweight. The coupler should apply a coupling force over a large area, and should be simple to install and not require extensive wiring.
The present invention is directed to an improved coupler for coupling two bodies. The coupling force is applied over a relatively large area. The coupler is compact, lightweight and does not require extensive wiring. Structural elements and actuator elements of the coupler are incorporated in a self contained, integrated unit.
A coupler having features of the present invention includes an outer member mounted about an inner member for relative motion on mutually engaged concentric bearing surfaces. The two members are assembled together such that the surfaces engage each other with an interference fit that resists relative motion and frictionally couples the members.
One member includes a piezoelectric component having inner and outer walls. The surface of the one member is located on an inner or outer wall of the piezoelectric component. The surface on the piezoelectric component moves towards or away from the other surface when a voltage difference is applied across the inner and outer walls of the piezoelectric component and increases or decreases the interference fit. The frictional coupling of the two members is selectively controlled by varying the voltage difference applied across the walls of the piezoelectric component to increase or decrease the interference fit between the two surfaces.
The coupling force is generated over a relatively large surface area and efficiently couples the members. Only one piezoelectric component is needed to couple the two bodies and so the coupler is lightweight, compact and easily wired to a voltage source.
In possible embodiments of the present invention, only two electrodes are required to wire the piezoelectric component to a voltage source. One electrode is electrically connected to the component outer wall and the other electrode is electrically connected to the component inner wall. The need to wire a large number of piezoelectric bodies is eliminated and the weight of the coupling is reduced.
In particularly advantageous embodiments, the other member is formed from an electrically conductive material, such as steel. The conductive material forms part of the electrical connection between the wall of the piezoelectric component engaging the conductive member and a terminal of a voltage source. This allows an electrode to be easily attached to an exposed surface of the conductive member.
In preferred embodiments of the present invention the piezoelectric component forms a portion of the inner member. The outer member presses inwardly and compresses the piezoelectric component. Conventional piezoelectric materials are stronger in compression than in tension, and so it is mechanically advantageous to have the piezoelectric component inside the outer member. However, if the piezoelectric material is sufficiently strong in tension, the piezoelectric component could form the outer member. In such embodiments the interference fit could tension the piezoelectric component.
In an advantageous embodiment of the present invention, the coupler forms a portion of a controlled collapse joint that varies the coupling between two bodies in response to some triggering event. The outer member is connected to one body and the inner member is connected to the other body. The joint enables the controlled coupling of one body with respect to the other body in response to external events to advantageously regulate the coupling of the two bodies during the event.
The piezoelectric component is a piezoelectric cylinder that forms a portion of the inner member. The outer member is a steel cylinder mounted on the piezoelectric cylinder for rotation, translation or combined rotation and translation about or along the piezoelectric cylinder. The outer and inner walls of the piezoelectric cylinder are electrically connected to a variable voltage system that forms a portion of a control system that actively controls the coupling of the coupler.
The control system includes a computer that controls the output of the voltage source and a sensor that transmits data to the computer enabling the computer to resolve the triggering event. The computer varies the coupling of the two bodies in response to the triggering event.
Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings illustrating the invention, of which there are four sheets of drawings and two embodiments.